Port height



  • Is there any benefit to raising the cylinder with a base riser and machining it down to raise port height or is this not recommended thanks jimmy



  • Yes, it can be done. But you need to know what you're doing if you want to head down that route.

    The theory, raising the exhaust port height is going to raise peak power, at the sacrafice of bottom end.

    The problem being that if you raise the barrel, you're going to drastically reduce compression.

    This might be acceptable, if bottom end is really not needed and you are going to advance the ignition so much that the current compression would cause the engine to detonate. So under this assumption you're looking at runnin avgas 130 and not pump petrol. Seeing as you're asking the question and not answering, I'm going to assume you will be running pump fuel.

    Secondly, Because you're raising the barrel and not the exhaust port, you're going to alter the porting timings for all windows, including boost ports, to the engine. Since the actual compression is not the same as the theorectical compression, your swept volume will be less and therefore you will actually be reducing displacement.

    This could be rectified by utilising a longer stroke, alter the cylinder head and the squishband and you may see noticeable gains.

    But the drawback is two strokes do not like over square engines and Yamaha did a smashing job designing the cylinder.

    If you look at any modern performance engine of recent recent times, say KTM, you will actually notice the inverse. The ports are way low down deep in the bore, not raised. And instead the head is recessed into the bore. This will alter the port timings to allow for that midrange punch KTM's are notorious for, without sacraficing that peak power (hence why new KTM 125's are pushing way over 30 bhp at the wheel.

    All of this work is great, if you have supporting mods to go with it. Or, look at it another way, these mods are pointless if you don't have an ignition system to compensate for the changes you've made.

    I always recommend that if you're going to start tuning a two stroke, start with the ignition. Otherwise you won't truly be able to reap the rewards, since the stock ignition retards at certain RPM's, making it unfeasible to obtain that potential power.



  • @Calum A bit off the original topic I know but......
    i've seen a lot of chat saying the dt ignition is pretty poor and hence a programmable cdi is a great place to extract more power. Out of interest how much does a zeeltronic retail for - can't seem to find a price anywhere.

    Also noticed on the dt you get a midrange punch that quickly signs off and although the engine revs out it doesn't pull as hard above a certain rpm although it will keep revving further, in your experience does advancing the timing eliminate that characteristic of the engine? I appreciate others might not notice this drop off in power but I am spoilt riding ktm's on a regular basis!



  • @2Stroke4Smoke it’s around the 200 pound mark



  • @2Stroke4Smoke No, that sounds like the powervalve retarding to me. It should pull hard to the red line when derestricted.

    The ignition on the DT is a reliable tune. An aftermarket ignition (for me) didn't do so much to the top end as it does to the mid range. Much punchier throughout the rev range. No doubt peak power was increased, but it was definitely the midrange I was impressed with.

    I just bought a Zeel for my RS from PJME at a cost of £300.

    I paid £160 plus VAT for my DT one years ago.

    It's a worth while upgrade, great place to start if you're looking for extra ponies, but don't compare it to a KTM as it simply ain't that.